Improving AirTightness:
Tightening the Air Barrier

Various home performance programs set their own air tightness targets, and it’s measured in different ways.  The air leakage rate usually compared with the volume or surface area of the house.

Southface Energy Institute’s Home Performance with ENERGY STAR program sets air tightness limits for existing buildings as about a 20% improvement unless the house almost tight or very air leaky.

The EarthCraft House program in the Southeast sets air leakage at a maximum of 0.5 CFM50 per square foot of building envelope: walls, floors, & ceilings.

Customer Performance Targets for Air Sealing

In general, a house should be made moderately tight, about 0.35 natural air changes per hour (0.35 ACHnat).  Currently, new houses are at about 0.5 ACHnat.  Leaky homes are range around 0.75 – 1.50 ACHnat

Preparing the House for Air Sealing

Once decisions have been made, a weatherization contractor air seals the house using a process called air sealing.  However, before starting air sealing, ventilation, combustion, moisture control issues should be corrected first.

Correct Ventilation & Moisture Issues. Until now, the house may have been satisfying combustion safety. Indoor air quality, and moisture control needs with the air leaks.  Air sealing can cause water heaters and furnaces to pull smoke back into the house.   It can also allow moisture to accumulate in the house, promoting mold and other microbial growth while otherwise rotting the building.  Indoor air pollutants can get trapped, leading to breathing problems.

Correction Methods. These conditions are correctable by providing adequate and balanced air to water heaters and furnaces and adding mechanical ventilation is appropriate places, usually bath area and rangehood vents.  Sources of water and water vapor need to be eliminated or controlled.  Depending on how tight the house is expected to be when air sealing is complete, other ventilation types may need be installed.

Air Sealing the House

After combustion air, moisture exchange, and ventilation issues are addressed.  Air sealing can begin. 

Set Air Sealing Limit. It’s unrealistic and would be incredibly expensive to make the house completely air tight.  Set air sealing priorities and follow them until objectives are met.

In-Process Air Leakage Testing. During air sealing activities, air infiltration testing should be done periodically to check progress.  Often the building tightness limit, financial target, or some other goal is set to determine when air sealing should stop.             

Air Sealing Priorities. In general, we propose the following air sealing process:

  1. Stop major air movement across walls, floors, & ceilings
  2. Fix and seal ductwork (discussed separately)
  3. Seal or fix big air leaks, wherever they are
  4. Seal the attic (ceiling or roofline)
  5. Seal the foundation (wall or floor above)
  6. Seal exterior walls

Reduce Convection. Limiting air movement across wall, floors, & ceilings greatly reduces heat gain or loss through though surfaces, and has a huge impact on comfort and utility bills.

Stop Duct Leaks. Reducing air leakage through ductwork outside the house limits air leakage when the heating & cooling system is off and on.  Air leakage through ductwork can be more than 2-3 times worse when the equipment is running.

Big Air Leaks First. Any big air leaks that can be stopped without tearing apart the building or by providing reasonable access needs to be done, wherever it is.  Air sealing floor assemblies between floors usually needs to wait until the flooring is changed, because the subfloor needs to be opened.  There’s also a lot of air leakage under baseboards that can be stopped when flooring is changed too. 

Air Seal Attic. Once these things are complete, our preference is to seal the attic first to stop air movement due to stack effect.  Air escaping through the ceiling pulls air in from floors areas the hardest.  Limiting airflow through the ceiling can greatly reduce the amount of outdoor pollutants and moisture drawn into the house.  This can be done by either foaming the roofline or sealing holes through the ceiling.

Seal Foundation Area Air Barrier. After the attic, the foundation area is sealed, either the foundation wall or overhead floor assembly.  Beside stopping air leaks, the goal is to limit moisture migration into the house as well as soil gases.

Air Seal Exterior Walls. The remainder of the house is the walls.  The most uncomfortable air leaks are around windows and doors.  Window frames are sealed to the wall.  Weatherstripping is installed around doors. 

On newer houses, it’s not cost effective to seal the remainder of the wall because it would require removing siding or wallboard to do it.  However, if the wall cavities are empty, filling walls with blown insulation does limit airflow.

 Post Air Sealing Testing.  When the weatherization contractor is done air sealing, final testing is needed, sometimes by a third-party who didn’t do the work.

For home performance contractors, final testing is mandatory when there are combustion appliances are present.  The concern is ensuring the areas fire places, water heaters, and furnaces are in are not depressurized to the point of causing smoke to backdraft into the house.

Where there are incentives or certificates offered, the home performance program sponsor usually requires final testing too.  Here, their considerations are to verify air tightness targets have been met.

We are ready, willing, and able to perform third-party testing.

Here’s to better indoor air quality and energy efficient house!

In Conclusion . . .

After controlling moisture flow into the house, providing adequate ventilation, and ensuring there is plenty of combustion air for water heaters & furnaces, the most cost effective thing to do for leaky houses is to air seal the walls, ceilings, & floors and then insulate them properly.

The financial result should be lower energy bills.  The health benefits include cleaner air, possibly resulting in fewer doctor visits.  From a home maintenance point of view, there is less dust to clean up and remove from equipment.  Clean heating & cooling equipment runs more efficiently.


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